1. Field of the Invention
The present invention relates to a device for coding a picture signal by compression and, more particularly, to a picture signal compression coding device which maintains the amount of compressed data representative of a picture constant at all times.
2. Description of the Prior Art
Digital picture data representative of a picture picked up by an electronic still camera, for example, are stored in a memory. Various kinds of compression coding schemes have been proposed to reduce the amount of such digital picture data and thereby the required memory capacity. Among them, a two-dimensional orthogonal transform coding scheme is extensively used because it codes data by a large compression ratio and because it allows a minimum of picture distortions particular to coding to occur.
Two-dimensional orthogonal transform coding is such that picture data representative of a single picture are divided into a plurality of blocks, and the picture data are subjected to two-dimensional orthogonal transform block by block. The picture data having undergone orthogonal transform, i.e., transform coefficients are compared with a predetermined threshold so as to discard those transform coefficients which are lower than the threshold. The remaining data are normalized and then written to a memory card or similar recording medium. The coded data outputted by the above procedure differs in amount from one picture data to another and, therefore, they cannot be written to a memory without resorting to troublesome processing.
In light of the above, there has been proposed a system which calculates the degree to which high frequency components are predominant, i.e., an activity picture by picture and selects a normalizing coefficient in matching relation to the calculated activity. By so selecting a normalizing coefficient, it is possible to maintain the amount of data constant throughout all the pictures.
Color picture data is made up of a luminance signal component Y and chorominance signal components Cr and Cb. Usually, more coded data are allocated to the luminance signal component Y than to the chrominance signal components Cr and Cb. The coded data outputs of the luminance signal component Y and chrominance signal components Cr and Cb each consist of coded data of DC components and coded data of AC components which have undergone orthogonal transform. The amount of AC coded data is determined by substracting the amount of DC coded data from the amount of coded data allocated to the associated component, i.e. Y or Cr and Cb.
Therefore, concerning the data of chrominance signal components Cr or Cb, for example, the allocation of coded data which is comparatively small in amount, it sometimes occurs that the amount of DC coded data of each of the components Cr and Cb exceeds the amount assigned thereto. Then, the amount of coded data of the DC components becomes short even if all the amount of coded data assigned to the component of interest are used. This prevents the DC coded data from being outputted and thereby practically disables the compressing coding device.